Unaudited Condensed Consolidated Income Statements - USD ($) shares in Millions, $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
TOTAL REVENUE		$ 5,259.9	$ 5,139.4
TOTAL COST OF SALES (excluding depreciation shown below)		4,417.6	4,319.6
TOTAL GROSS PROFIT		842.3	819.8
Selling, general, and administrative expenses		626.8	595.3
Depreciation and amortization		40.0	37.3
Other income, net		(10.3)	(19.5)
OPERATING INCOME		185.8	206.7
Non-operating income (expense) items:
Floorplan interest expense		(28.3)	(21.5)
Other interest expense		(32.3)	(28.8)
Interest income		0.2	0.4
Other income, net		0.8	3.0
INCOME FROM CONTINUING OPERATIONS BEFORE INCOME TAXES		126.2	159.8
Income tax provision		32.9	61.6
NET INCOME FROM CONTINUING OPERATIONS		93.3	98.2
Income (loss) from discontinued operations, net of income taxes		0.4	(0.1)
NET INCOME		$ 93.7	$ 98.1
BASIC EARNINGS (LOSS) PER SHARE:
Continuing operations (in dollars per share)	[1]	$ 1.01	$ 0.97
Discontinued operations (in dollars per share)	[1]	0.00	0.00
Net income (in dollars per share)	[1]	$ 1.02	$ 0.97
Weighted average common shares outstanding (in shares)		92.1	101.1
DILUTED EARNINGS (LOSS) PER SHARE:
Continuing operations (in dollars per share)	[1]	$ 1.01	$ 0.97
Discontinued operations (in dollars per share)	[1]	0.00	0.00
Net income (in dollars per share)	[1]	$ 1.01	$ 0.97
Weighted average common shares outstanding (in shares)		92.7	101.6
COMMON SHARES OUTSTANDING, net of treasury stock, at period end (in shares)		91.3	101.3
New Vehicle [Member]
TOTAL REVENUE		$ 2,802.3	$ 2,796.2
TOTAL COST OF SALES (excluding depreciation shown below)		2,672.6	2,651.9
TOTAL GROSS PROFIT		129.7	144.3
Used Vehicle [Member]
TOTAL REVENUE		1,330.5	1,240.9
TOTAL COST OF SALES (excluding depreciation shown below)		1,244.7	1,163.9
TOTAL GROSS PROFIT		85.8	77.0
Parts and Service [Member]
TOTAL REVENUE		858.5	845.1
TOTAL COST OF SALES (excluding depreciation shown below)		473.0	474.1
TOTAL GROSS PROFIT		385.5	371.0
Finance and Insurance, Net [Member]
TOTAL REVENUE		240.8	221.6
TOTAL GROSS PROFIT		240.8	221.6
Other Goods and Services [Member]
TOTAL REVENUE		27.8	35.6
TOTAL COST OF SALES (excluding depreciation shown below)		27.3	29.7
TOTAL GROSS PROFIT		$ 0.5	$ 5.9

[1] Earnings per share amounts are calculated discretely and therefore may not add up to the total due to rounding.